Some conventional systems and methods for accomplishing these goals rely upon scan mirrors that receive signals from an object and relay them into an aperture of an optical system—and, for response, conversely receive signals from the optical-system aperture and return those signals toward the object. Some such systems and methods instead (or also) rely upon gimbals that support and reorient the entire optical system. Both approaches entail relatively high moments of inertia, and accordingly large motors and elevated power requirements.
Such configurations require extremely adverse tradeoffs and compromises between, on one hand, undesirably high cost and size, and on the other hand structural weaknesses that lead to unreliability and even failure. For instance expensive custom parts and instrumentation are the rule rather than the exception, while some conventional devices have dimensions on the order of one to ten centimeters with mass of one to ten or even hundreds of kilograms.
These are significant handicaps for—in particular—devices that may be for use in airplanes and satellites. Even in these cases, such drawbacks might be acceptable if such systems provided superb performance, but unfortunately angular resolution in conventional systems of various types is generally no better than two-thirds of a degree—sometimes as coarse as ten degrees and more.
For example gimbal controls are most typically good to roughly one degree or less, although some units capable of precision in tens of microradians are available for millions of dollars each. Sensors using focal-plane arrays, e. g. quad cells, are typically precise to roughly ten degrees. Other nonmechanical systems include quad cells behind fisheye lenses.
The poor angular resolution and other performance limitations of such sensors arise in part from use of fixed, very large sensor assemblies, typically quad cells, CCD or CMOS arrays, at a focal plane—with fixed fields of view. These components accordingly also suffer from limited fields of regard. Furthermore the necessity for downloading into a computer memory the massive volumes of data from multimegabyte sensor arrays makes the frame rate of these systems extremely slow.
In efforts to improve the field of regard, the large areal arrays are sometimes placed behind radically wide-angle lenses, even fish-eye lenses. This strategy, however, is counterproductive in that it only compounds the data-download problem, while also yielding intrinsically coarse angular resolution and very nonlinear angular mapping.
In other words these systems are squeezed between the need for high resolution and the need for broad field of regard; this squeeze comes down to an all-but-prohibitive demand for dynamic range, or bandwidth. Data congestion, furthermore, is doubly problematic because in these systems the entire contents of every frame must be retrieved before that frame can be searched for an optical source of interest.
One rather unnoticed contributor to inadequate dynamic range is the direct relationship between gimbal angle or scan-mirror angle and excursion of the beam in the external scanned volume. That relationship is a natural one-to-one for a gimbal system, and one-to-two for a rotating mirror. Since the direct effect of mechanical rotation is relatively slow for gimbals, and relatively limited in overall angular excursion for scan mirrors, the external beam-angle excursion is either slow or limited, or both.
In attempts to mitigate low resolution and frame rate, some workers have proposed to substitute a so-called “position-sensing detector” (PSD) for the commonly used larger arrays. The advantage of a PSD—which is a unitary device, not an array—is that it inherently locates and reports position of only a detected optical source, not an entire scene, and thus requires download of only a far smaller amount of data.
Another inherent advantage of a PSD is that it provides a continuous, analog positional readout, intrinsically yielding extremely high resolution. The report from an array is instead quantized by the pixel (or “aliasing”) effect that is central to any kind of array detection.
The PSD reports position on its own sensitive surface, in units of distance from its nominal center along two orthogonal axes. To find angular mapping, typically these off-center coordinates are divided by the focal length of a final focusing element.
Unfortunately these reported distances and therefore the angular mapping of a PSD are nonlinear, to the extent of several percent at the PSD edges—aggravating the analogous handicap introduced by a fish-eye or other wide-angle lens—and are also temperature sensitive. The detector may report accurately that an optical source has been sensed, but fail to report accurately where that object is, unless it is near the nominal center, or origin of coordinates.
It might be supposed—although in actuality this supposition is well beyond the present state of the art, and artisans of ordinary skill—that such a system could be quickly turned to look directly at the candidate object, for a more-accurate assessment of position. In any conventional detector, however, this solution is impractical due to the lumbering response of an associated gimbal system, or even of a scan mirror that is redirecting the light into the detector aperture.
Often it is desirable to know something more about an optical source that has been noticed—the character of the light itself, and any intelligence signal that may be impressed upon that light. Accurate determination of wavelength and frequency modulation information, as may be gleaned from the foregoing discussion, is beyond the capabilities of these systems. Similarly infeasible is any exploration of physical objects that may be associated with the optical source.
The intractability of attempting to operate with such systems may be clarified by consideration of some practical situations which call for use of optical sensors. In most applications a person or an apparatus points a light source toward, most typically, some sort of vehicle—to guide an object in an attempt to rendezvous with the vehicle. Commonly the intention is adversarial, as for example damage to the vehicle; while the optical-sensor apparatus is mounted on the vehicle and its purpose is to detect the presence of the light beam and initiate some protective response.
Such response, usually intended to produce confusion as to the exact location of the vehicle, sometimes takes the form of returning a literally blinding flash of light toward the person or apparatus that is pointing the original source, to temporarily dazzle and confuse that source-controlling entity. Alternatively a response can be to eject from the vehicle many particles that strongly reflect the guide light, to instead confuse directional-control mechanisms of the moving object. Accompanying either of these may be an entirely different kind of response, namely an effort to disable the source-pointing person or apparatus, or the object. Such a disabling response, directed toward the object or source, may take the form of either a physical article or of powerful radiation. Still another desirable kind of response would be investigatory, i.e. determining the character of the guide beam or of the guided object; such information can be used to determine and report the nature of the guiding system itself, either for purposes of immediate efforts to confuse and avoid or for future protective-design work.
The person or apparatus pointing the source may be adjacent to the initial position of the object. In a sense this is the easiest case from the standpoint of protective response, because the source can be treated as a beacon for guidance of a disabling response that eliminates both the light source and the object—if the response is sufficiently prompt, so that the source and object are still not only in-line but also relatively close together. In another sense, however, this is a difficult case from the standpoint of confusion, because the object may have been designed to look (for its guidance) backward at the source rather than forward at the vehicle—in which event the ejection of reflecting particles cannot confuse the directional-control mechanisms of the object, as long as the pointing entity can keep the vehicle in view.
The person or apparatus pointing the source may, however, instead be at a different position—off to the side from the path of the object, and from a line between the source and the object. In this event, disabling both the source and object with a single response is not possible; but at least confusion can be more-readily produced since the object is necessarily designed to look forward at the vehicle, so that either the dazzling or the decoy-particle strategy, or both, can be effective.
One type of movable-mirror device that is known in various kinds of optical-detection systems is a single scan mirror of about 25 or 30 mm or more, consistent with the earlier statement of dimensions for conventional systems. Such mirrors are too bulky and heavy to overcome the previously discussed problems of response speed.
Another type of known movable-mirror device is a spinning cylinder with multiple mirrors carried on its outer surface. Such a polyhedral construction does provide a movable mirror, sometimes disposed along an optical path between a detector and an entrance aperture. Dimensions of each of the mirrors in such a device are typically in the tens of millimeters, also consistent with the previous indication of representative dimensions for conventional systems. Hence the overall device and even the individual mirrors are too big and heavy to free the optical-detection art from the response-speed and related limitations discussed above. These mirror wheels are ordinarily made to spin continuously; hence the individual mirrors of such an array lack independent maneuverability for customized control movements. Accordingly they are poorly suited for practical use in rapid detection and tracking of a particular source object.
Also of interest are telescopes—including astronomical telescopes—particularly of the type that has a movable mirror positioned between an entrance aperture and a detector. For present purposes, however, any interest in such devices is academic, as the movable components are relatively huge and far too massive to be useful in any rapid-response system. Even more relevant is the typical limitation of field of view, in telescopes, to less than ten degrees.
Smaller deformable mirrors, too, are sometimes placed within optical systems in positions such as just described. A device of this type generally comprises a continuous reflective membrane that is controllably bent and distorted to correct wavefront errors. Such mirrors are typically at least 20 to 30 mm across.
Another type of known moving-mirror device, never heretofore associated with the field of optical-source detection that is under consideration here, is called a “microelectromechanical system” (MEMS) mirror. Such devices, introduced some years ago by the Texas Instruments Company, and more recently in versions produced by Lucent Technologies and called an “optical switch”, most commonly take the form of arrays of very small mirrors—each on the order of ten to 500 microns across. At least in principle individual mirrors can be made available in the same format. In use these devices, while some are capable of continuous positional control, are most often only bistable, used for switching in optical information networks and also in an image-projection system for personal computers.
Other familiar optical devices, not previously associated with the present field, are afocal lens packages used e.g. as lens focal-length extenders. These are commonplace in ordinary cameras.
Almost all the optical devices discussed above, and most conspicuously the astronomical ones and MEMS devices, are known only in different fields from the present invention.
As can now be seen, the related art fails to resolve the previously described problems. The efforts outlined above, although praiseworthy, leave room for considerable refinement.